The present invention concerns a method for making devices, particularly microelectromechanical systems, and devices made by the method.
Microtechnology-based energy and chemical systems (MECS) are devices that rely on embedded microstructures for their function. The overall size of MECS devices places them in the mesoscopic regime, i.e. in a size range between macro objects, such as automobile engines and laboratory vacuum pumps, and the intricate MEMS based sensors that reside on a silicon chip. Mesoscopic systems are expected to provide a number of important functions where a premium is placed on mobility, compactness, and/or point application. The internal processes of these devices rely on length scales that are much smaller than traditional systems. For thermal and chemical applications, a small characteristic size provides the benefits of high rates of heat and mass transfer, large surface-to-volume ratios, and the opportunity for operating at elevated pressures.
For other more mechanically operated meso machines, such as generators and motors, small dimensions imply rapid response and compact design. Furthermore, these systems often can be volume produced, which results in a substantial reduction in the cost of producing each device. MECS also will find increasingly important uses where small-scale heat engines, heat pumps and refrigerators are needed. For example, the development of miniature refrigerators could provide point cooling of high speed electronics and communication equipment for enhancing performance (Little, 1990). Also, power packs based on combustion rather than electrochemistry could extend operating times of electronic devices by a factor of ten (Benson and Ponton, 1993). MECS also can be used for chemical processing. For example, miniaturized chemical reactors could provide on-site neutralization of toxic chemicals, thereby eliminating the need for transport and burial (Koeneman, et al., 1997).
Many MECS devices rely on fluidic processes. As a result, the same technology can be applied to biological applications. Miniaturized bioreactors could provide precisely regulated environments for small groups of cells to enhance their production of therapeutic drugs, or the detection of toxic compounds. Such bio-applications could range from benchtop research to large-scale production facilities.
A specific example of a microtechnology-based energy and chemical system is a heat pump for cryogenic cooling of high-speed electronics. One embodiment of such a heat pump is a resonantly coupled alpha-Stirling cooler, which is only 2 centimeters long. The heat pump comprises a thermal compressor, which further includes a displacer, an electromagnetic coil, and a heating element. The displacer comprises a series of linear flexural springs and spacers laminated together. The displacer is fixed in the center. When a magnetic field is applied, the outside of the displacer oscillates, thereby producing a Stirling cycle cooling effect. The alpha-Stirling cooler includes a heat exchanger/regenerator, having multiple microchannels. The performance of this system depends on several microfeatures, including the microchannels, which are about 50 xcexcm wide, and the gap between the displacer and the cylinder, which is about 20 xcexcm.
The manufacturing technology currently available for producing microelectronics or MEMS will not be sufficient for producing MECS because of the mechanical and thermal requirements of these systems. For example, it is desirable to make the displacer springs in the alpha-Stirling heat pump from a highly fatigue resistant, ferromagnetic material. This is not possible with current MEMS fabrication technology.
It also is desirable to fabricate high-aspect-ratio microstructures, for instance a microstructure having a height-to-width ratio of at least 20:1. Currently, the MEMS fabrication technologies that are capable of producing these high-aspect-ratio microstructures are not suitable for economical, high-volume production. Some, such as LIGA, are quite expensive. Lower-cost MEMS fabrication technologies are being developed, but they generally rely on lithographic techniques to form micromolds for electroplating metals. The drawbacks to these lithographic techniques include limited material selection, limited geometric complexity, and inconsistent pattern-transferring methods. The non-lithographic fabrication technologies that currently are available are either serial in nature or involve single-layer thin-film formation. Serial techniques are unlikely to allow economical mass production. Single-layer, film-forming techniques provide limited geometrical complexity.
Thus, current MEMS fabrication technology cannot produce the geometries necessary for MECS in a low-cost, high-volume manner. Thus, there is a need for economical microassembly methods to allow low-cost, high-volume production of MECS sufficient to compete with conventional macroscale energy and chemical systems. For example, it is desirable to have components in these miniature systems that are freely movable within the system. There also is a need for new and improved methods for bonding together laminae to form monolithic MECS.
The present invention addresses the needs identified in the Background, such as the need for economical microassembly methods to allow low-cost, high-volume production of MECS sufficient to compete with conventional macroscale energy and chemical systems. Moreover, the method of the present invention can make miniature systems having components that are freely movable within the system, and also provides a new and improved method for bonding together laminae to form monolithic MECS.
The present invention comprises a method for making devices in a pre-assembled state. The method comprises providing plural laminae, at least some of which include structures and substructures that define components of devices, registering the laminae, bonding the laminae, and dissociating the components to make the device. Component dissociation can be performed prior to, subsequent to, or simultaneously with bonding the laminae. One aspect of the invention involves providing plural laminae where at least one lamina comprises at least one structure, at least one substructure, and at least one fixture bridge. Fixture bridges are used to couple the structures and the substructures across the spaces between these structures and substructures in the pre-assembled state. The device is fully assembled once the laminae are bonded together and the substructures are dissociated from the structures. The substructures are dissociated by eliminating the fixture bridges, either prior to or subsequent to registering and bonding the laminae. In another aspect of this invention, fixture bridges can be used to couple two substructures, two non-adjacent substructures, or can be used within a substructure.
Various methods can be used to eliminate the fixture bridges. One method comprises applying an electrical current across the fixture bridge of sufficient power to eliminate the fixture bridge. Another method for dissociating the substructures comprises heating the fixture bridge, and selectively dissolving the heated fixture bridge with a chemical. The fixture bridge is heated to a temperature sufficient to allow the chemical to selectively dissolve the fixture bridge. A third method involves ablating the fixture bridge with a laser. UV lasers are preferable because such lasers minimize heat affects and open up the technique to non-metal materials, such as ceramics and polymers.
Another embodiment of this invention comprises making an array of devices, such that at least one of the plural laminae has an array of at least two assemblies. The assemblies comprise at least one structure, at least one substructure, and at least one fixture bridge. The devices are assembled by eliminating the fixture bridges either prior to or subsequent to registering and bonding the plural laminae. The array of devices can be used in parallel, or the individual devices can be dissociated from each other.
In another aspect of this invention, the laminae are selectively bonded to each other at specific sites on the laminae by microprojection welding or diffusion bonding. Microprojection welding comprises forming lamina with projections that extend from at least one planar surface thereof. The projections can be made of the lamina material or another material suitable for welding or brazing. Selective bonding is accomplished by first placing the laminae between electrodes, such that the laminae contact one another at the site of the projections, and then passing a current through the electrodes. Thus, the laminae are bonded together selectively at the sites of the projections.
In another aspect of the invention, the laminae can be bonded using a diffusion soldering method. Diffusion soldering first comprises preparing and plating the surface of each lamina to be bonded. A typical plating process involves placing a relatively thin first layer (approximately 0.5 xcexcm) on a bare surface of a lamina that will receive the layer. This first layer promotes adhesion of other platable metals. Then, a second, generally thicker layer, such as a layer preferably from about 2 to about 5 xcexcm thick, is plated over the first layer as a base upon which to plate a third layer having a thickness preferably from about 2 to about 5 xcexcm. Working embodiments used a laminate stack having alternating surfaces plated with either silver or tin. The two outside laminae were plated with silver so that the final, bonded stack did not adhere to the alignment jig. Copper may be preferred as a bonding agent because of its ability to readily bond to both nickel and either silver or tin, materials found useful for diffusion soldering of laminae. The temperature of the laminate stack being diffusion bonded/soldered is then momentarily raised above the melting point of the law-temperature melting metal, generally tin (232xc2x0 C.), under a compression pressure sufficient to achieve the bond. Working embodiments have used compression pressures of approximately 2 MPa to about 5 MPa. Diffusion soldering provides a quick and efficient method for forming a bonded laminate stack having strong bonds that are resistant to moderately high temperatures without restricting flow in or through internal features.